Many studies in the last ten years have shown the key role of melatonin (N-acetyl-5-methoxytryptamine) in many physiopathological phenomena and in the control of circadian rhythm. Its half-life is, however, quite short owing to its being rapidly metabolised. It is thus very valuable to be able to provide the clinician with melatonin analogues that are metabolically more stable, that have an agonist or antagonist character and which may be expected to have a therapeutic action that is superior to that of the hormone itself.
In addition to their beneficial action on circadian rhythm disorders (J. Neurosurg. 1985, 63, pp. 321-341) and sleep disorders (Psychopharmacology, 1990, 100, pp. 222-226), ligands of the melatoninergic system have valuable pharmacological properties in respect of the central nervous system, especially anxiolytic and antipsychotic properties (Neuropharmacology of Pineal Secretions, 1990, 8 (3-4), pp. 264-272) and analgesic properties (Pharmacopsychiat., 1987, 20, pp. 222-223), and also for the treatment of Parkinson's disease (J. Neurosurg. 1985, 63, pp. 321-341) and Alzheimer's disease (Brain Research, 1990, 528, pp. 170-174). Those compounds have also shown activity on certain cancers (Melatonin--Clinical Perspectives, Oxford University Press, 1988, pp. 164-165) ovulation (Science 1987, 227, pp. 714-720), diabetes (Clinical Endocrinology, 1986, 24 pp. 359-364), and in the treatment of obesity (International Journal of Eating Disorders, 1996, 20 (4), pp. 443-446).
Those various effects take place via the intermediary of specific melatonin receptors. Molecular biology studies have shown the existence of a number of receptor sub-types that can bind that hormone (Trends Pharmacol. Sci., 1995, 16, p. 50; WO 97.04094). It has been possible to locate some of those receptors and to characterise them for different species, including mammals. In order to be able to understand the physiological functions of those receptors better, it is very useful to have available specific ligands. Moreover, by interacting selectively with one or other of those receptors, such compounds can be excellent medicaments for the clinician in the treatment of pathologies associated with the melatoninergic system, some of which have been mentioned above.
The compounds of the present invention have a novel structure that is characterised by a naphthylalkyl-amide, -thioamide, -urea or -thiourea ring structure that is partially hydrogenated or unhydrogenated and has two substituents on the main ring structure. Surprisingly, that structure imparts to the compounds very great affinity for melatonin receptors and selectivity for one or other of the receptor sub-types.